What is this? it may be RV related, you never know!

One way to find out,,,,,,,,,,get a lug nut, tie a string to it,,,,,lower down the pipe,,, see if anything is on the nut when you pull it out,, How far from the house is it? That cap definitely looks to be a Home Heating oil fill cap! Had the same type on the oil fill tube when I lived in Philly!!

Oh,, might want to put on a pair of disposable gloves when you lower that nut into the tank,,